4,4-methylene-bis-(N-phenylalkylcarbamate) is a precursor to 4,4-diphenylmethanediisocyanate (hereinafter referred to as pure MDI) which is useful for the preparation of a polyurethane and in various other expanding fields of application. For instance, pure MDI is used for the preparation of polyurethane elastomer or spandex, or in the coating process of artificial leather; and crude MDI which can be formed from 2,4-methylene-bis-(N-phenylalkylcarbamate) and trimer or tetramet thereof, etc., is used as an insulator for such products as refrigerator, air conditioner, etc., an engineering plastic material for such automobile components as steering wheel and bumper or as a synthetic wood material.
Conventionally, 4,4-methylene-bis-(N-phenylalkylcarbamate) is prepared by reacting N-phenylalkylcarbamate with a methylating agent such as formalin in the presence of an acid catalyst and a suitable solvent, as shown in the following scheme 1: ##STR1## wherein:
R is a lower alkyl group having 8 or fewer carbon atoms, and
n and m are independently an integer of 1 to 5.
However, the above process yields a large amount of isomeric products, e.g., 2,4-methylene-bis-(N-phenylalkylcarbamate) and trimer or tetramer thereof, etc., (or polymethylene polyphenylalkylcarbamate) and undesired N-benzyl compounds, e.g., N-(alkoxycarbonyl, phenylaminomethylphenyl and trimer or tetramer thereof, etc., in addition to the desired product of 4,4-methylene-bis-(N-phenylalkylcarbamate); and, consequently, an additional procedure for isolating the by-products is required. When the reaction is carried out in water as a solvent, it produces a large amount of undesired N-benzyl compounds due primarily to a reaction occurring relative to a nitrogen atom of N-phenylalkylcarbamate.
It has further been proven very difficult to selectively separate 4,4-methylene-bis-(N-phenylalkylcarbamate) alone from the products produced by condensation of N-phenylalkylcarbamate with a methylating agent by using a conventional process such as distillation since some of the products, e.g., 4,4-methylene-bis-(N-phenylalkylcarbamate), 2,4-methylene-bis-(N-phenylalkylcarbamate), polymethylene polyphenylalkylcarbamate and N-benzyl compounds, have similar physical properties and high boiling points and, moreover, are unstable at a high temperature.
For instance, Japanese Laid-open Patent Application No. Sho 59-172451 discloses a process which comprises condensing N-phenylalkylcarbamate with a methylating agent such as formalin, trioxane or paraformaldehyde in the presence of an inorganic liquid acid, e.g., hydrochloric, sulfuric or phosphoric acid as a catalyst and water as a solvent. However, it has the disadvantage that large amounts of by-products, e.g., N-benzyl compounds, are produced; and, as a result, the desired compound cannot be obtained in a sufficient selectivity. Further, it is difficult to separate and recover the desired compound in a commercial scale because the product is combined with the by-products to form large solid chunks.
Japanese Laid-open Patent Application Nos. Sho 55-129260, 57-171953 and 58-62151 and U.S. Pat. No. 4,162,362 are directed to various condensation processes conducted in the presence of various Br nsted acids comprising trifluoromethanesulfonic acid, sulfonic fluoride, carboxylic acid, etc., or various Lewis acids comprising iron chloride on graphite, boron trifluoride, etc., as a catalyst; and nitrobenzene, benzene and sulfolane as an organic solvent. However, these processes suffer from the common deficiency of producing a low yield of 30 to 50% of 4,4-methylene-bis-(N-phenylalkylcarbamate) owing to, e.g., the high acidity of the catalyst used and a large amount of polymethylene polyphenylalkylcarbamate, which aggravates the task of selectively preparating 4,4-methylene-bis-(N-phenylalkylcarbamate).
The processes disclosed in Japanese Laid-open Patent Application Nos. Sho 60-237058 and 60-231640 employ a mixture of methylene-(N-phenylalkylcarbamate) and polymethylene polyphenylalkylcarbamate pyrolyzed at a high temperature in the presence of a suitable solvent and a catalyst to obtain a mixture of 4,4-diphenylmethanediisocyanate and polymethylene polyphenylisocyanate; and the isocyanate mixture is evaporated in a thin film evaporator at a high temperature under a vaccum for the prevention of polymerization of the products to separate a portion of 4,4-diphenylmethanediisocyanate as described in British Patent Nos. 1398975 and 1520055. These processes have the similar defect of a low selectivity for 4,4-diphenylmethane diisocyanate and require expensive equipments, complicated techniques and a high consumption of energy to meet the requirements of maintaining high temperatures and vaccum condition.
As a result, needs have continued to exist for the development of a process for the preparation of 4,4-methylene-bis-(N-phenylalkylcarbamate) in a high selectivity, with a reduced level of by-products and at a lower cost.